Delta-sigma modulators (noise shapers) are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, a delta-sigma modulator spreads quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
In addition to data conversion applications, delta-sigma noise shapers are increasingly utilized in the design of digital amplifiers. In one particular technique, a digital delta-sigma noise shaper provides a noise shaped (quantized) digital data stream to a pulse width (duty cycle) modulated (PWM) stream, which in turn drives a linear amplifier output stage and associated load. This technique is generally described in U.S. Pat. No. 5,784,017, entitled “Analogue and Digital Convertors Using Pulse Edge Modulators with Non-linearity Error Correction”, granted Jul. 21, 1998, and U.S. Pat. No. 5,548,286, entitled “Analogue and Digital Convertor Using Pulse Edge Modulators with Non-linearity Error Correction”, granted Aug. 20, 1996, both to Craven, U.S. Pat. No. 5,815,102, entitled “Delta Sigma PWM DAC to Reduce Switching”, granted Sep. 29, 1998, to the present inventor (incorporated herein by reference), U.S. patent application Ser. No. 09/163,235 to Melanson (incorporated herein by reference), and International Patent Application No. PCT/DK97/00133 by Risbo.
One problem, which occurs in multiple-channel PWM systems, such as those used in multiple-channel DACs, is cross-coupling of energy between the associated data paths when the outputs of two or more PWM stages switch simultaneously or nearly simultaneously. The result is noise and distortion in the PWM output signals, especially when the data input to the PWM stages have a low signal level, such as during quiet periods in a digital audio data stream being converted to PWM.
Hence, given the increased use of multiple-channel PWM systems, new techniques are required for minimizing distortion and noise in multiple PWM streams as the result of time-proximate output switching of the corresponding PWM stages.